The Intermediate-Range Nuclear Forces (INF) treaty is expected to formally collapse on August 2 after both Russia and the United States pulled out, dealing a blow to international arms control efforts.

Stoltenberg said there was "no indication" the Kremlin was prepared to destroy a missile system the West says breaches the INF and so NATO defence ministers will use talks on Wednesday and Thursday to agree countermeasures.

"Our response will be defensive, measured and coordinated. We will not mirror what Russia does," Stoltenberg told reporters, saying NATO did not intend to deploy new land-based nuclear missiles in Europe.

"We do not want a new arms race, but as Russia is deploying new missiles, we must ensure that our deterrence and defence remains credible and effective."

With US-Iran tensions running high, NATO defence ministers will get their first chance to meet the new Acting US Defense Secretary Mark Esper, who will brief them on last week's dramatic confrontation in the Gulf.

Stoltenberg deflected questions about whether the US informed NATO it was considering air strikes against Iran -- eventually called off -- in response to the shooting down of an American surveillance drone.

- End of the road for INF -

After years of complaining to Russia about the 9M729 ground-launched cruise missile system, the US announced in February it would pull out of the INF in August unless Moscow backed down.

NATO's 29 allies have unanimously backed Washington's assessment that the nuclear-capable missile violates the 1987 treaty, which banned ground launched missiles with a range of 500 to 5,500 kilometres (300 to 3,400 miles).

Diplomats say that rather than simply agreeing with the US, several allies have carried out their own independent investigation and reached the same conclusion themselves.

A meeting of the NATO-Russia Council has been called next week as a last-ditch bid to persuade Moscow to abandon the missiles and save the treaty.

"We call on Russia to take the responsible path," Stoltenberg said.

"But unfortunately, we have seen no indication that Russia intends to do so. In fact, it continues to develop and field the new missiles."

After long denying the existence of the missile system, Russia now insists it complies with the INF and accuses the US in turn of violating the accord.

US defence experts have been researching new systems to respond to the Russian missile since Washington suspended the treaty in February, US Ambassador to NATO Kay Bailey Hutchison said.

"We do not have the defence yet, we don't have exactly what we're going to do yet," she said, adding that "all options are on the table", though -- in a nod to European concerns -- only with conventional weapons.

"Beyond that there has not been any testing or final decision on exactly what kind of defence we would prepare for all the missiles we know Russia has."

- Spend, spend, spend -

The thorny issue of defence spending will also be on the ministers' plate as they look ahead to a potentially explosive leaders' meeting in London in December.

President Donald Trump has repeatedly berated NATO allies, especially Germany, for not doing enough to meet the agreed target of trying to spend two percent of GDP on defence, accusing them of freeloading on the US.

Preliminary figures for 2019 published Tuesday showed eight NATO countries on course to hit the target -- just one more than 2018.

Overall defence expenditure by Europe and Canada is up 3.9 percent, the fifth consecutive year of increased spending.